You'll need nerves of titanium and the reflexes of a Targothian panther as the invaders attack you at warp speeds. However, your concentration must be at it's peak when taking on the major sentries at the end of each of the four missions.

Every once in a while, a bomb power-up will appear on the corner of the screen to be shot to destroy all
enemies, or a friendly ship will drop off an item to restore shields
(but don’t shoot the ship!). Boss battles are also fast and furious
goings, but the aforementioned depleting of shields for every shot
quickly makes this an unforgiving experience, since there is little
ramp up on the difficulty before being thrown into the deep end.
Unfortunately, the shortcomings of the single-fire Zapper controller
cannot sufficiently keep up.

To the Earth feels like a tech demo that was hastily repeated across four monotonous stages and slapped onto a cartridge to meet deadlines. Some people might enjoy its unique brand of gameplay but for me it's not the kind of balls-out shooter that the Zapper does best.